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The effect of a period of rest (Tp) lasting from 5 to 120 s on the action potentials (AP) of the ventricular myocardium of 9- to 11-week human embryos was studied. The result was shortening of the AP proportional to the duration of the pause and it was accompanied by a shift of the AP plateau phase to more negative membrane voltages. Shortening of the AP during the pause was more pronounced in solutions with a half extracellular calcium concentration. Recovery from the effect of the pause took place significantly more slowly in solutions with the lower extracellular calcium concentration than in the presence of a normal concentration.

Using the glass microelectrode technique we measured the effect of adrenaline (ADR, 10(-5) mol . l-1) and acetylcholine (ACh, 10(-5) mol . l-1) on action potentials (AP) of the ventricular myocardium of 8- to 11-week-old human embryos. ACh prolonged the AP (by 20%), without affecting voltage of the plateau phase. ADR markedly elevated the plateau phase (by 8 mV) and prolonged the AP (by up to 40% of the initial duration). The ACh effect attained the maximum in 5-7 min and was frequency dependent. The ADR effect reached the maximum in 1-2 min. Neither ACh nor ADR affected the resting membrane potential value. The effect of ACh was blocked by atropine (10(-6) mol . l-1), showing that it is mediated by way of M-receptors. Propranolol (6.10(-6) mol . l-1) only partly blocked the effects of ADR. The results show that the human ventricular myocardium is already sensitive to the action of ACh and ADR before autonomic innervation of the heart develops.

The basic electrophysiological manifestations of the ventricular myocardium of twelve 7- to 12-week human embryos were measured with a glass electrode and a programmed stimulation technique. The resting membrane potential value was 79.37 +/- 0.34 mV and the overshoot 32.7 +/- 0.57 mV; the action potential (AP) duration at 1 Hz stimulation frequency was 120.0 +/- 5.7 ms at AP plateau phase levels and 258 +/- 17 ms at the level corresponding to 95% repolarization. The duration of the AP was a function of the stimulation frequency. i.e. it altered in correlation to the stimulation programme fully developed frequency sensitivity). In stimulation with different frequencies the duration of the steady state AP was in an inverse relation to the stimulation frequency, the maximum changes being found in the terminal repolarization zone. An interpolated extrasystole mainly affected the duration of the plateau phase.

Using the glass microelectrodes techniques, we analysed the causes of the shortening of the ventricular myocardial action potentials (AP) of albino rats during postnatal development. Delayed outward currents were blocked with tetraethylammonium (TEA) in 20 mmol.l-1 concentration. TEA led to prolongation of action potentials and to accentuation of frequency sensitivity in all the given age groups, but a block of TEA-sensitive currents nevertheless does not permit the conclusion that the cause of postnatal AP shortening is due to an increase in the intensity of outward currents. Slow inward current (Isi) were blocked with verapamil (2.10(-5) mol.l-1), which markedly shortened the myocardial AP of newborn rats; with advancing age its effect diminished and shifted to the negative membrane voltage level. The Isi was stimulated by adrenaline (10(-5) mol.l-1), which markedly prolonged the AP of newborn animals; with advancing age its effect rapidly diminished and was virtually undetectable in 10-day-old animals. The results indicate that postnatal AP prolongation is caused by a drop in the Isi rate rather than by the growth of outward repolarizing currents.

To elucidate the postnatal development of electrogenesis in the rat ventricular myocardium, we investigated specimens of the right ventricles of rats of different ages (newborn, 2 days, 5 days, 10 days and adult) by means of glass microelectrodes and a programmable stimulator. The resting membrane potential rose from -82 mV (newborn) to -89.6 mV (adult). During the first 10 postnatal days the plateau phase of the action potential (AP) shortened from 46 ms to 16 ms; in adulthood its duration was 9.6 ms. An increase in the stimulation frequency from 2 Hz to 10 Hz led to pronounced shortening of the myocardial AP of newborn and 2-day-old rats; with advancing age the influence of a mounting frequency diminished and the myocardium of adult rats was frequency-insensitive. The first AP evoked after a rest interval in the myocardium of newborn animals was prolonged proportionally to the logarithm of the duration of the interval; the AP of adult rats was unaffected by the length of the interval. During the rest interval diastolic depolarization appeared on the myocardium of newborn rats; with advancing age it slowed down and the decrease in membrane voltage was smaller. In the adult rats myocardium there was no diastolic depolarization.

Experiments were carried out on the working myocardium of the right heart ventricle of newborn and adult rabbits, guinea-pigs, dogs and albino rats. In the dog, the guinea-pig and the rabbit, after ten action potentials (AP) elicited with 1 Hz frequency we always interpolated an extrasystole at an interval (TE) of 100-900 ms. In albino rats we used a basic frequency of 2 Hz and a TE of 30-370 ms from the last regular AP. Using glass microelectrodes, we recorded the extrasystolic AP (EAP) and the next subsequent AP (2AP). The results were evaluated by constructing graphs of the correlations of the duration of the plateau phase (D0) to TE and of the duration of repolarization to -60 mV level (D60) to the TE. In the myocardium of newborn rabbits, guinea-pigs and dogs, with short TE both D0 and D60 of the EAP are shorter than in the steady state (SS), while for the 2AP the same parameters are influenced only a little. As the TE lengthens, the EAP gradually acquire a length corresponding more to the SS. With TE longer than half the duration of the cycle in the steady state the EAP return to normal, while the 2AP become shorter. The effect of extrasystole on the rat EAP and 2AP diminished with advancing age. In the myocardium of adult rabbits and adult guinea-pigs, and slightly in the myocardium of adult dogs and newborn rats, we observed that the duration of the EAP, with certain TE, was greater than in the steady state.(ABSTRACT TRUNCATED AT 250 WORDS)